Gus van Sant is one of Cannes's favourite sons: his 2003 film Elephant went home with the Palme d'Or, while his 2007 contender, Paranoid Park, was awarded a one-off 60th anniversary prize. But every son has the potential to be wayward, and with Restless, Van Sant has gone right off the path. To spare any blushes, the Cannes selectors have wisely decided to divert it away from the main competition, and place it in the Un Certain Regard ("worth a look") section, where its underwhelming performances, limp direction and background-hum irritation won't be exposed to quite as much scrutiny.

Restless, essentially, is Van Sant's take on the adorably quirky awkward-teen comedy of the Juno/Rushmore/Submarine mould. Shot on his home turf in Portland, Oregon, it focuses almost exclusively on two school-age protagonists: Enoch, played by Henry "son of Dennis" Hopper in his first sizeable role, and Annabel, played by the somewhat more established Mia Wasikowska. Enoch, as you might have guessed from his unfashionable name, is the film's main vortex of quirk: first seen lying on the ground, chalking an outline around his own body like a crime scene cop, he is a serial impostor at funerals, mopes around in graveyards, plays battleships with the ghost of a second world war kamikaze pilot.

But unlike the death-obsessed kid in Harold and Maude, Enoch at least hits it off with someone his own age. At one random cremation, he runs into Annabel, who peeps at him adorably from under the brim of a cloche hat. She soon whips it off to reveal a Mia Farrow-style crop – which, though it's never made clear, could actually be the result of treatment for a brain tumour. As it unspools, Restless turns out to possess that most slippery and dislikable of modern movie-making tics: the healthy-seeming cancer patient. Wasikowska, whose character is supposed to be dying from said tumour, never looks less than fresh as a daisy at any point: even when she's going into fits or hooked up to a transfusion line.

Restless's main problem is that, although barely a minute of screen time goes by without some adorable quirk revealing itself, it's essentially dramatically inert. For all their soppy looks and histrionic yakking, there's very little chemistry between the leads, which might have made up for it. Hopper certainly resembles his dad in his Rebel Without a Cause period, but has a long way to go before matching Hopper Sr's screen charisma: his Enoch comes across as snotty, petulant and self-involved by turns. And Wasikowska, though undeniably one of Hollywood's major hopes, can't do much with a character so gamine it hurts.

As for Van Sant, you can sense what he was trying to do: there are few better directors than him at capturing the raw confusion of adolescence. But he's moved away from the blank-effect style so effective in Elephant and Paranoid Park, and turned to a more conventional, emotional key. Could this be the influence of producers Ron Howard and Brian Grazer, two of Hollywood's more safety-first big-shots? Who knows. Whatever the cause, this labours when it should float.